In internal combustion engines, the sealing of the combustion chamber by the pistons and piston rings is not absolute. Throughout the working cycle, gases pass from the cylinder into the crankcase. These gases--below called blow-by gases or BB gases for short--must be able to escape from the crankcase. This is usually effected by providing an orifice on the valve cover or engine block.
Protection of the environment demands prevention of an escape of oil from the engine. In naturally aspirated engines, the oil mist blown out of the crankcase (BB gases) is passed back into the induction side of the engine, so that external oil contamination is avoided. In fact, this measure has become a statutory regulation for gasoline engines in many countries. Corresponding proposals have the aim of introducing this technique also in the diesel engine.
In some countries, regulations provide that, in spark-ignition engines, a reduced pressure must prevail in the crankcase throughout the load and speed range, in order to avoid an escape of oil through the seals of the crankcase. In principle, only a slightly reduced pressure should prevail in the crankcase throughout the operating range, since, the greater the reduced pressure, the more oil is disadvantageously extracted together with the BB gas.
It is then customary to return the BB gases into the induction line of the internal combustion engine upstream of the air filter or to pass them into the open through an oil separator. In the case of a return of the BB gases into the induction line, some motor manufacturers see an advantage in valve lubrication, but others do not utilise the BB return for this purpose.
The oil separator can be integrated in the valve cover or it can be inserted as an individual component. It filters out a large proportion of the oil contained in the BB gas before the BB gas passes into the open or is returned into the engine--that is to say, fed to combustion.
When the oil separators nowadays available on the market are used, however, a considerable quantity of oil still breaks through. The requirement of establishing a reduced pressure in the crankcase hitherto did not permit the use of oil separators having a higher degree of purification of the BB gas. In fact, higher oil separation causes a greater pressure drop in the filter, so that the desired reduced pressure in the crankcase is no longer ensured.
For an internal combustion engine supercharged by a turbo-charger, printed publication GB-A 2 006 329 has disclosed the extraction of the BB gases from the crankcase with the aid of a pump system designed as an ejector or jet pump. The propellant used here is preferably supercharging air. The BB gases to be extracted from the crankcase flow through a control device which is placed in the extraction line and which controls the reduced pressure desired in the crankcase. The extracted BB gases are then returned into the system of the internal combustion engine. Such a circuit has the aim of obtaining vigorous extraction by means closely connected with the engine.
However, this presupposes that the propellants employed possess sufficient stored energy. Under full load, this is generally the case, but it is not the case when the engine runs under part load. In such cases, the supercharging pressure can hardly maintain the extraction of the BB gases from the crankcase, even if the control device is by-passed.
If, furthermore, an oil separator were to be provided downstream of a pump system, it would no longer be possible to establish the desired reduced pressure in the crankcase.